1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same. In particular, the invention relates to a substrate having an SOI (silicon on insulator) structure in which a semiconductor layer is formed by slicing a crystalline semiconductor substrate and bonded to a different kind of substrate. More particularly, the invention relates to a bonding SOI technique and a method for manufacturing an SOI substrate in which a single crystal or polycrystalline semiconductor layer is bonded to a substrate having an insulating surface such as a glass substrate. Furthermore, the invention relates to a display device or a semiconductor device having such an SOI structure.
2. Description of the Related Art
Instead of silicon wafers manufactured by slicing a single crystal semiconductor ingot, semiconductor substrates called silicon on insulators (SOI substrates) in which a thin single crystal semiconductor layer is provided over an insulating layer have been developed and are becoming widely used as substrates in manufacturing microprocessors and the like. This is because integrated circuits using SOI substrates attract attention since the parasitic capacitance between a drain of a transistor and a substrate can be reduced, which results in improved performance and reduced power consumption of the semiconductor integrated circuits.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known (for example, Reference 1: U.S. Pat. No. 6,372,609). According to the hydrogen ion implantation separation method, hydrogen ions are implanted into a silicon wafer to form a microbubble layer at a predetermined depth from the surface, and the microbubble layer is used as a cleavage surface to bond a thin silicon layer (SOI layer) to another silicon wafer. In addition to heat treatment for separating the SOI layer, it is necessary to perform heat treatment in an oxidizing atmosphere to form an oxide film over the SOI layer, and after the oxide film is removed, it is necessary to perform heat treatment at 1000° C. to 1300° C. in a reducing atmosphere to increase the bonding strength.
On the other hand, there has been an attempt to form an SOI layer over an insulating substrate such as a glass substrate. As an example of an SOI substrate in which an SOI layer is formed over a glass substrate, it is known that a thin single crystal silicon layer is formed over a glass substrate having a coating film by a hydrogen ion implantation separation method (Reference 2: Japanese Published Patent Application No. H11-163363, and Reference 3: U.S. Pat. No. 7,119,365). Also in that case, hydrogen ions are implanted into a piece of single crystal silicon to form a microbubble layer at a predetermined depth from the surface, and the piece of single crystal silicon is bonded to a glass substrate and then separated with the microbubble layer used as a cleavage surface, whereby a thin silicon layer (SOI layer) is formed over the glass substrate.
A thin film transistor (TFT) formed over an insulating substrate such as a glass substrate has a structure in which a base insulating film, an active layer, a gate insulating film, a gate electrode, an interlayer insulating film, and a wiring are formed over the glass substrate. In order to increase the operation speed of a TFT, the design rule of the whole TFT has been reduced.